To study the interaction of opiates and of endogenous opioid peptides with the several opiate receptors by the use of four parallel assays: the guinea-pig ileum, mouse vas deferens and the inhibition of binding of (H3-duhydromorphine and of (H3-enkephalim. To investigate the usefulness of these assays for the design of new opiates and of analogues of opioid peptides which may interact preferentially with one or the other of the opiate receptors. To study the differential distribution in the central nervous system of micron-opiate receptors, represented by (H3)-dihydromorphine binding sites, and delta bar-opiate receptors, represented by (H3)-enkephalin binding sites. To study the biosynthesis of methionine-and leucine-enkephalin and define the nature and characteristics of their immediate precursors. To study the conditions of the release of the enkephalins. To continue the investigation of the role of the interaction between endogenous opioid peptides and exogenously applied narcotic analgesics in the development of tolerance and dependence.